The pyrethroids with which the present invention is concerned are crystallizable esters having at least one asymmetric carbon atom to which an epimerizable proton is attached. These pesticidally active pyrethroids additionally contain at least one and usually two or more other asymmetric carbon atoms and therefore comprise isomeric mixtures wherein one or more of the isomers are more pesticidally active than the others. Representative of such pyrethroids are the alpha-cyanobenzyl esters of the formula (A): ##STR1## wherein R.sup.1 is halogen, haloalkyl, alkenyl or haloalkenyl; each R.sup.2 independently is halogen, alkyl, haloalkyl, alkoxy, phenyl, phenoxy, phenylalkyl, substituted phenyl and substituted phenylalkyl wherein the substituents include one or more of alkyl, halogen, haloalkyl, nitro, hydroxy and cyano; and n is 0-5, preferably 1-3. In the above formula the asymmetric carbon atoms are marked 1, 3 and alpha (.alpha.). All of the substituents on a host group may be the same, or the substituents may be different. Alkyl and alkoxy may contain 1-8 carbon atoms, preferably 1-4 carbon atoms. Alkenyl may comprise 2-8 carbon atoms, preferably 2-4 carbon atoms. Halogen includes fluorine, chlorine and bromine. A typical phenylalkyl group is benzyl. Substituted phenyl includes tolyl, xylyl, trichlorophenyl and trifluoromethylphenyl. Substituted phenylalkyl includes methylbenzyl, trichlorobenzyl and trifluoromethylbenzyl.
The foregoing and other pyrethroids are well known as disclosed, for example, in Kirk-Othmer, Encyclopedia of Chemical Technology, Second Edition, Vol. 13, pages 456-458, in the following U.S. Pat. Nos:
4,024,163--Elliot et al (NRDC) PA1 4,133,826--Warnant et al (Roussel Uclaf) PA1 4,136,195--Warnant et al (Roussel Uclaf) PA1 4,213,916--Davies et al (Shell) PA1 4,287,208--Fuchs et al (Bayer) PA1 4,308,279--Smeltz (FMC) PA1 4,427,598--Mason et al (Shell) PA1 4,512,931--Robson (ICI) PA1 4,544,508--Fuchs et al (Bayer) PA1 4,544,510--Van Berkel et al (Shell) PA1 4,560,515--Stoutamire et al (Shell) PA1 4,582,646--Stoutamire et al (Shell) PA1 4,670,464--Doyle et al (ICI) PA1 4,681,969--Williams et al (ICI) PA1 WO 86/04215--Hidasi et al (Chinoin) PA1 WO 86/04216--Hidasi et al (Chinoin)
and in the following PCT patent publications:
All of the listed patents and publications are incorporated herein by reference.
Preferred pyrethroids convertible to more active isomers in accordance with the present invention are those of formula A wherein R.sup.1 is dihalovinyl or a tetrahalopropenyl group such as 3,3,3-trifluoro-2-chloropropen-1-yl, R.sup.2 is phenoxy, and n is 1 or 2. The more preferred pyrethroids are those wherein n is 1, R.sup.1 is dihalovinyl or tetrahalopropenyl and R.sup.2 is phenoxy; and those wherein n is 2, R.sup.1 is dihalovinyl or tetrahalopropenyl and one R.sup.2 is fluorine and the other R.sup.2 is phenoxy. The preferred compounds are isomeric mixtures having the common name "cyfluthrin" when R.sup.1 is dichlorovinyl, n is 2 and one R.sup.2 is 4-fluoro and the other R.sup.2 is 3-phenoxy. When R.sup.1 is dichlorovinyl, n is 1 and R.sup.2 is phenoxy, the preferred mixtures have the common name "cypermethrin."
Cypermethrin contains four cis and four trans isomers designated I-VIII as follows: